I am using Adafruit IO+ account and confused with '60 data points per minute' data rate. What is data point? Does it mean I can read/write at the most 60 MQTT requests per minute across the account irrespective of number of feeds?

Adafruit IO imposes a rate limit on all data modification APIs to prevent excessive load on the service. If a user performs too many data create, update, or delete actions in a short period of time then the system will start rejecting requests.

If you have a free Adafruit IO Account, the rate limit is 30 data points per minute.

If you have upgraded to an Adafruit IO Plus account, the base rate limit is 60 data points per minute.

If you exceed this limit, a notice will be sent to the {username}/throttle MQTT topic. You can subscribe to the topic if you wish to know when the Adafruit IO rate limit has been exceeded for your user account. This limit applies to all Data record modification actions over the HTTP and MQTT APIs, so if you have multiple devices or clients publishing data, be sure to delay their updates enough that the total rate is below your account limit.

One data record modification is any action that creates, updates, or deletes a single Data record.

I read this as applying to activity across your account, as it includes the phrase for your user account.

Oh no! I thought limit is per feed and hence I applied for io+ subscription. So if I have multiple devices publishing to different mqtt topics per second, it never works and I will get throttle error. If this limit is per account then let's assume I have 2 devices posting to 2 individual feeds once a second won't work (60+60 requests from two devices). It is such a low rate that Adafruit mqtt broker cannot be used for even two feeds, why then unlimited feed creation has been provided I don't understand, it will never work.

So if I have multiple devices publishing to different mqtt topics per second, it never works and I will get throttle error.

As long as the devices combined are not publishing a combined 60 data points per minute, you will not receive a throttle error.

let's assume I have 2 devices posting to 2 individual feeds once a second won't work (60+60 requests from two devices).

You'd need to decrease the data rate. Publishing every second is not a common data rate for most IoT applications. This rate of publishing data from an IoT device/project would decrease the device's battery life since the wireless radio will need to turn on, send data, and shut down.